A typical internal antenna for wireless devices, like for example cell phones, consists of a conductive plate or wire usually mounted on a plastic carrier that provides mechanical support. The antenna is assembled in the wireless device, forming an integral part of such a device. The wireless device will usually have a multilayer printed circuit board (PCB) on which it carries the electronics.
In order to feed the antenna, an electrical path must exist to connect the antenna to the Radio Frequency (RF) front-end of the circuit, or the RF input/output of an electronic device, on the PCB. Said electrical path is created through contact means which ensure the electrical connection of the antenna to the RF front-end of the circuit.
A typical way to feed the antenna is by means of a spring contact. The spring contact ensures good electrical continuity of the signal from the RF signal tracks on the PCB to the antenna, which is achieved by tensional strength of the lever of the spring contact on the appropriate pad or contact region on the PCB.
Furthermore, the spring contact has also the mechanical function of providing robustness of the assembly in front of tolerance errors in the height of the antenna over the PCB when the piece that contains the antenna is fixed onto the PCB, for example by means of clips, screws or adhesives.
FIG. 2 shows a typical prior-art compression spring contact.
As shown in FIG. 2, the interference of the tip 22c of the spring contact 22 with the second conducting surface 21 (typically a PCB) translates the vertical displacement necessary to achieve a given tensional strength on the pad of the second conducting surface 21, into horizontal displacement 26 on the plane of the second conducting surface 21. The behavior of the spring contact 22 is such that when compression is applied to the spring contact 22 the entire spring lever 22b reacts mainly as if it rotated with respect to the center of curvature of the first bent 23 of the spring contact 22 after departing from the first conducting surface 20 (typically an antenna element) to a new position 25. Since the center of curvature of this bent 23 is closer to the first conducting surface 20 than to the second conducting surface 21, and hence far from the tip 22c of the spring contact 22, even a rotation by a small angular amount of the lever 22b of the spring contact 22 results in significant linear displacement 26 on the plane of the second conducting surface 21. This implies that the pad on the second conducting surface that accepts the tip 22c of the spring lever 22b has to be long enough in the direction of the displacement of the spring contact 22 in order to ensure that the tip 22c of the spring contact 22 lands on the pad, and thus good electrical contact is obtained.
The extra space necessary for the pad that accepts the spring contact becomes a serious overhead when the size of the PCB of the wireless device is particularly small (as for example those in slide-type or clamshell-type cell phones), and/or high density of components is needed to host the electronics and other elements like for instance integrated circuits, batteries, handset-cameras and speakers, LCD screens, or vibrators.
There exists one state of the art solution that attempts to solve this problem, and that is the use of a POGO pin. A POGO pin is a component that ensures the electrical connection of the antenna to the RF module of a wireless device featuring a reduced contact area. This type of component has a number of disadvantages. POGO pins are more expensive than conventional compression spring contacts and do still require a certain contact area, which is not always available in PCBs with high density of components. Another disadvantage is that a POGO pin has to be considered as an additional component that has to be taken into account at the early stage of PCB design. That is a serious drawback for antenna designers since the antenna design is often carried out after the design of other parts of the wireless device such as the PCB has been closed.